A measure for those who want to go straight (4)

I believe the answer is:

rule

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This clue could be a double definition.

'a measure' is the first definition.
(I know that rule is a type of measure)

'for those who want to go straight' is the second definition.
The definition suggests a singular noun which matches the answer.

Can you help me to learn more?

(Other definitions for rule that I've seen before include "Decree" , "Govern or control" , "Dominate" , "Dominion" , "Prescribed guide for conduct" .)

I've seen this clue in The Irish News.

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